WASHINGTON
— By pushing through a prescription drug benefit for Medicare recipients after years of congressional gridlock, President Bush has fulfilled a key campaign promise - and given himself and the Republican Party a potential boost with what could be the most critical voting bloc in 2004: senior citizens.

The bill represents the most sweeping change to Medicare in the program's 38-year history. Depending on how it plays out, it could alter the nation's political landscape by giving Republicans a major stake in an issue Democrats have claimed ownership of for decades.

More broadly, it could wind up reshaping the political allegiance of one of the most potent voting groups in America - one that, almost since the advent of Medicare, Social Security, and other federal entitlement programs, has been a solid Democratic constituency.

"If the Democrats don't win among seniors, they don't win the election, pure and simple," says Glen Bolger, a GOP pollster. "Democrats are gambling that this is a law that seniors ultimately won't like, and that they'll take it out on the supporters of it. But in gambling on that, they're also running the risk that seniors will say, 'Look, we shouldn't let the perfect be the enemy of the good - and this is a good bill.' "

Certainly, how the issue plays out will depend in part on the practical impact of the bill as it takes effect over the course of the next year and beyond. Some seniors may be dismayed to learn that the bulk of the benefit won't take effect until 2006, and Democrats argue the measure could backfire, causing some seniors to lose their current drug coverage, and raising premiums for others. Over the long term, Democrats say the bill will endanger Medicare overall by putting it into competition with private insurers.

But Republicans and some neutral observers say that while the bill may be imperfect, for many seniors the simple fact that Congress has finally passed something, after years of promises, constitutes a victory. And if the new drug benefit increases Mr. Bush's support among seniors by even a small margin, it could significantly bolster his chances of reelection.

Neither party is underestimating the importance of the senior vote, which often plays a critical role in elections, since older voters have a higher and more reliable turnout rate than any other group.

"They're the tried and true backbone of the electorate," says Susan MacManus, a political science professor at the University of South Florida, and author of "Targeting Senior Voters." "They will show up no matter what."

The shifting senior vote

And while seniors once tilted decidedly Democratic, they have evolved in recent years into one of the most hotly contested swing votes in the nation.

Much of the shift is simply generational: The oldest seniors, who were shaped by the FDR years and tend to be more dependent on government entitlements, still lean Democratic. But those under 70 are more likely to have spent their formative years under Eisenhower. They tend to be wealthier, and are often willing to align themselves with the GOP.

In the 2000 election, 47 percent of people over 60 voted Republican, compared with 44 percent in 1996. In the 2002 midterm elections, Republicans won the senior vote by five points. Significantly, several top swing states being targeted in 2004 by both parties - such as Florida and Pennsylvania - also have high concentrations of older voters, which could give them even more sway in the outcome.

To some extent, Professor MacManus says, the senior vote increasingly resembles that of the nation at large. "The electorate is divided - and that division holds true among seniors as well, up to around age 70," she says.

Indeed, early reactions of senior voters to the bill often seem to reflect party affiliation more than anything else. In Green Valley, Ariz., a retirement community south of Tucson, for instance, sentiment is clearly mixed. Republican Ray Greeley believes his party is doing the right thing. "I don't think retirees would have it as good today under Clinton," he says. "We need to change the Medicare system one way or another, and I think Bush will leave it in good shape."

But Democrat Sandra Stone is worried. "I think the Republicans are trying to privatize something that doesn't need to be privatized," says the retired high school teacher. "The Democrats are at least trying to keep Medicare going."

Pocketbook may drive politics

Still, the bottom line may ultimately trump party loyalty, if seniors wind up getting some relief on their prescription drug bill. Among many older voters, the prevailing sentiment is: "Politicians can argue about the minutiae and the long-term consequences and everything, but we want relief and we want it now," says MacManus.

Aside from their feelings about the details of the bill, many Democrats and Republicans agree that something had to be done, given the escalating cost of prescription drugs. In Green Valley, Democrat Ellie Kurtz says insurance from her husband's former employer covers the cost of prescriptions, but acknowledges not everyone has that advantage.

"We're lucky," she says. "It's terrible for a lot of people, many of them paying $500 or more at the pharmacy every month."

At the sprawling senior center in Garner, N.C., residents had so many concerns about the bill that they sent a letter to Sen. Elizabeth Dole (R). Director Torrey Blackmar, who spends hours helping seniors fill out Medicare and Medicaid forms, believes that the benefit will prove confusing. "A lot of people are going to think they're covered, and find out they're not," she says.

Despite the bill's flaws, Ms. Blackmar adds that's it's "probably a good thing" overall. Sidney Jordan, a resident at the center who complains that the Democratic Party "left me behind a long time ago," says Republicans should get credit for having taken action - something Democrats didn't do when they were in power. "This is something Democrats have been talking about for a long time, but now the Republicans are the ones who approved it," he says. "At the very least, it's a step in the right direction."

Yet others are still withholding judgment. Retired state worker George Smith, who's filling up his Ford Ranger in the nearby town of Clayton, believes the benefit may ultimately prove disappointing. Although he agrees Republicans will get "a lot of mileage" out of the measure, he notes that with drug prices rising by as much as 15 percent a year, the full value of the benefits will have decreased substantially by the time they take effect in 2006. And he deems the overall $400 billion price tag too high for a country "that's already broke."